Ethylene-vinyl alcohol copolymers (hereinafter, sometimes abbreviated as EVOH) are excellent in, for example, gas barrier properties, oil resistance and aroma retention properties and therefore have been used in various applications. However, EVOH also has some drawbacks of high moisture permeability and high expense. In order to maintain advantages inherent in EVOH and compensate defects thereof, EVOH is usually used in lamination with a thermoplastic resin, such as polyolefin and polystyrene. However, because the adhesion of EVOH to such a thermoplastic resin is poor, it is necessary to form an adhesive layer between both layers. As such an adhesive, modified polyolefins, such as polyolefins (e.g., polyethylene, polypropylene, and ethylene-vinyl acetate copolymers) modified with maleic anhydride and ethylene-ethyl acrylate-maleic anhydride copolymers are used widely. However, when such an adhesive is used, adhesion may be insufficient, depending on the brand of EVOH. In some cases, the interlayer adhesion strength after coextrusion molding may change with time. On the other hand, since a resin containing functional groups of at least one kind selected from the group consisting of a boronic acid group and boron-containing groups each capable of being converted into a boronic acid group in the presence of water has a very high reactivity with EVOH, it is possible to solve the aforementioned problems by using it as an adhesive.
WO 02/060961, which is also published as EP1369438A, discloses a multilayer structure produced by laminating an EVOH layer and a polyolefin layer via a layer of an adhesive. As the adhesive, a resin composition containing a polyolefin and a styrene-hydrogenated diene block copolymer containing in its side chain at least one functional group selected from the group consisting of a boronic acid group and boron-containing groups each capable of being converted into a boronic acid group in the presence of water is used. It is shown that multilayer structures obtained in such a procedure have satisfactory interlayer adhesion. It is disclosed that the adhesive resin composition used therein can be produced by melt-kneading the styrene-hydrogenated diene block copolymer and the polyolefin using a Banbury mixer, a twin screw extruder, or the like. In Examples of the publication cited above, a multilayer structure is produced by melt-kneading the styrene-hydrogenated diene block copolymer and the polyolefin using a twin screw extruder to obtain an adhesive resin composition, and then feeding the resulting adhesive resin composition, a polyolefin and an EVOH into single screw extruders, respectively, followed by coextrusion molding.
Since a resin containing functional groups of at least one kind selected from the group consisting of a boronic acid group and boron-containing groups each capable of being converted into a boronic acid group in the presence of water is expensive, it is preferable, from the economical point of view, to use it by diluting it with an inexpensive polyolefin as disclosed in the publication cited above. However, the interlayer adhesion is not sufficient in some applications and, therefore, there is a strong demand for obtaining multilayer structures having satisfactory interlayer adhesion strength even if the used amount of the resin having a special functional group is reduced.